A conventional remote control system such as a vehicle entry control system controls a lock/unlock state of each door based upon the identification result of an ID code by inter-communication between a portable electronic key (a portable device) and an in-vehicle unit.
In such a system, an electric wave strength of a request signal transmitted from the in-vehicle unit or the like is adjusted in advance so that a reach range of the request signal is within a predetermined area in the circumference of the vehicle. As a result, a detection area of the portable electronic key is set in the circumference of the vehicle to always monitor an approach of a holder of the portable electronic key to the vehicle.
However, the request signal from the in-vehicle unit may be relayed by using a plurality of additional communication devices, making it possible to communicate the portable electronic key placed out of that area with the in-vehicle unit. In this case, even when a user carrying the portable electronic key is at a distance away from the vehicle, it is possible to unlock a door without the user's recognition.
In order to prevent this problem, German Patent No. 4020445 proposes a system, which is based on the finding that, in a case of enlarging a reach area of the request signal, a request time from a time point when the in-vehicle unit transmits the request signal to a time point when the in-vehicle unit receives the response signal from the portable electronic key increases. Therefore, in this system, a time limitation is imposed on receiving the response signal, so that the in-vehicle unit is adapted to receive only a response signal, which arrives within a waiting time corresponding to the detection area after the in-vehicle unit transmits the request signal. As a result, the door is designed not to be unlocked even if the in-vehicle unit receives the response signal at a time point exceeding the waiting time limitation.
However, imposing only a time limitation on receiving the response signal still has the following problem. That is, in a case where the plurality of the above communication devices can perform communication processing nearly without time delay, there is a possibility that the response signal from the portable electronic key at a remote area from the vehicle arrives within a waiting time of the response signal.